>> X-Envelope-From: <user@lanyon.com>
>> Received: from S253906HZ1EW06.usstls6-hosting.savvis.net (unknown
>> [209.16.192.170])
>>
>> Is it because there is no reverse DNS entry?
>
> As the documentation [1] clearly states, the second value (a) is a
> string matched against the relay's rDNS in the Received headers, and
> (b) it is your MX's responsibility to perform the rDNS lookup and add it
> to the header.
>
> The latter is key, because despite "no reverse DNS entry" being
> ambiguous, there actually is an rDNS entry for the IP. In DNS, it's
> merely missing from the Received header.
>
> $ host 209.16.192.170
> 170.192.16.209.in-addr.arpa domain name pointer Lanyon.com.
>
> So, fix your MX. :) With an rDNS entry in the header, you will need to

I'm not it's a problem with my system. It's not that IP that has the
problem, but with the host itself: